Superhard materials have a Vickers hardness (Hv), i.e. an indentation hardness, of at least 40 GPa and are widely used as abrasives for drilling, cutting, and other machining applications. Superhard materials often include boron, carbon, nitrogen and oxygen because these light elements have a small atomic radius and form strong and directional covalent bonds that produce tight, three-dimensional networks with extreme resistance to external shear.
Diamond is the hardest superhard material currently known, with an Hv of about 70-100 GPa. However, the actual performance of diamond as an abrasive is somewhat limited. Diamond is an unsuitable abrasive for machining ferrous alloys and has limited applications for high-speed cutting because it is converted into graphite in the presence of oxygen at temperatures over 800° C.
Cubic BN (cBN) is another important superhard material. While cBN is widely used for machining fully hardened steels and exhibits much better thermal stability than diamond, it is only about half as hard (Hv=45˜50 GPa) as diamond.
Superhard materials for industrial use are often in the form of sintered polycrystalline composites that incorporate microcrystalline grains of diamond or cubic boron nitride. The grains of this composite are tens to hundreds of micrometers in size, and usually include vacancies, dislocations, and other imperfections that multiply and propagate to form microcracks within individual crystals of a grain, and also along grain boundaries. As the microcracks grow, the materials deform and fracture.
Recently, a new class of materials known as superhard nanocomposites has been reported. Superhard nanocomposites contain superhard nanocrystalline grains embedded in an amorphous matrix. The amorphous matrix provides amorphous grain boundaries that absorb vacancies and dislocations, reduces the surface energy and residual stress among the grains, and permits the relaxation of mismatches between adjacent grains of different phases. While a number of superhard nanocomposites have been reported, no superhard nanocomposite bulk compact having the Vickers hardness of diamond has yet been prepared. Thus, there remains a need for a superhard nanocomposite compact with improved hardness, strength, and performance.
Therefore, an object of the present invention is to provide a bulk superhard nanocomposite compact with a high Vickers hardness.
Another object of the invention is to provide a method for preparing a bulk superhard nanocomposite compact with a high Vickers hardness.